Polyesters, particularly polyethylene terephthalate (PET) produced from ethylene glycol and terephthalic acid as starting materials, are used for a broad range of products such as containers, films, sheets, and fibers because such polyesters are superior in chemical and physical properties.
In recent years, a polyester obtained by copolymerizing this polyethylene terephthalate (PET) with 1,4-cyclohexanedimethanol (hereinafter abbreviated as CHDM) has drawn attention because of its superiority in transparency, impact resistance, formability, heat resistance and other parameters, and has been increasingly employed for various uses, particularly film (sheet), engineering plastic and other materials.
Similarly, a polyester obtained by copolymerizing polyethylene terephthalate (PET) with neopentyl glycol (hereinafter to be also abbreviated as NPG) has drawn attention because of its superiority in transparency and impact resistance as disclosed in JP-A-2000-109546.
Polyester can be produced from dicarboxylic acid or dialkyl ester thereof, and glycol as starting material monomers, by the steps of esterification reaction or transesterification reaction to give diester of dicarboxylic acid and glycol and/or oligomer thereof, and heating this product under reduced pressure to allow for polycondensation reaction until a desired degree of polymerization is achieved. In general terms, the thus—obtained polyester is subsequently continuously extruded from plural nozzles of a die in strands of molten resin from a reactor, cooled with a cooling medium such as water, cut in a suitable size (pelletized) and shipped as a starting material pellet (chip) for molding.
When the polyester chip has a high moisture content, the molecular weight of the polyester decreases in the subsequent melt extrusion step, and the subsequently formed products problematically have a lower mechanical strength. To avoid this in the production of polyester, polyester chips are generally dried in a dryer to give a final product. For drying polyester chips, a drying apparatus is generally used in view of drying efficiency, convenience and similar desirable properties.
However, as a result of the study of the present inventors, it has been found that, in the case of amorphous polyester chip wherein PET is copolymerized with 1,4-cyclohexanedimethanol, neopentyl glycol and the like, an increase in fine particles i.e., finely divided powder) is produced from the polyester chip during drying in a drying apparatus (e.g., batch type dryer, hopper type dryer with a stirrer etc.), and the finely divided powder exerts an adverse influence on the quality of subsequently formed products.
In other words, generation of fine particles causes segregation when mixing plural chips, which in turn poses difficulty in production of subsequently formed products having a uniform composition, resulting in quality variation of formed products. Furthermore, while the reason is unknown, moreover, it has also been found that a large content of fine particles degrades transparency.
As used herein, by “segregation” is meant, for example, a phenomenon wherein during a step of forming a mixture of two or more kinds of starting material chips, the presence of fine particles (finely divided powder) results in a chip mixture being unevenly distributed in a silo or hopper, causing inconsistent mixing ratio of the chip mixture during film forming, thereby resulting in greater variations of the polymer composition of the final film product. For example, when a segregated chip mixture is used for film production, the polymer composition of the film is deviated from uniform in the film running direction, which in turn causes problems of variation in the film properties (e.g., heat shrink property, mechanical strength etc.), inconsistent thickness and other undesirable properties.
In addition, when amorphous polyester chips are dried using a drying apparatus (e.g., batch type dryer, hopper type dryer with a stirrer etc.), individual chips are melted with each other and integrated due to the heat, and such melted chips degrade workability of drying process and subsequent forming process. The reason is that, although melting of chips does not occur readily in the case of crystalline polyesters such as PET, PBT, PEN and others, because polyester is crystallized by drying with heating, in the case of amorphous polyesters, crystallization does not occur.
Crude by-products resulting from the production process of polyester chips include large chips having a greater size than a specified size by-produced during pelletization (chip formation) and crude products resulting from the shape change of chips (film formation) during a drying step. It has been found that these are produced in a comparatively trace amount, and these alone do not readily give rise to a problem, however, once melted chips are produced, the chips cannot be taken out from the drying apparatus readily or chips are clogged in an input line, an inlet and the like in a forming machine during forming, and the forming process cannot be completed stably.
As regards drying of polyester chips, JP-A-2001-348425 proposes a method comprising washing polyester chip with water and drying by heating with the aim of removing oligomers such as cyclic trimer and the like from crystalline polyester.
The present invention has been made in view of the above-mentioned situation, wherein a first object is to provide an amorphous polyester chip capable of producing a subsequently formed product having a uniform polymer composition and a production method thereof, as well as a preservation method of the amorphous polyester chip. Particularly, the object is to provide an amorphous polyester chip capable of increasing processing operability during subsequent forming, and efficiently producing a subsequently formed product having a uniform polymer composition and a production method thereof, as well as a preservation method of the amorphous polyester chip.
A second object of the present invention is to provide an amorphous polyester chip capable of producing a subsequently formed product (particularly film) having a uniform polymer composition and superior in transparency and solvent adhesiveness, and a production method thereof, as well as a preservation method of the amorphous polyester chip.
A third object of the present invention is to provide an amorphous polyester chip having a uniform polymer composition and a high degree of polymerization, and capable of producing a film having superior mechanical strength, and a production method thereof, as well as a preservation method of the amorphous polyester chip.
A fourth object of the present invention is to provide an amorphous polyester chip having a uniform polymer composition and capable of efficiently producing a film superior in uniform thickness, and a production method thereof, as well as a preservation method of the amorphous polyester chip.